1. Field of the Invention
The present invention relates to a light-emitting element including an organic compound layer, and more specifically to an organic light-emitting element including at least one organic compound layer that is disposed between a pair of electrodes and includes a light-emitting layer emitting light by applying a current to the organic compound layer. The present invention also relates to a light-emitting device including the organic light-emitting element.
2. Description of the Related Art
Organic light-emitting elements (hereinafter simply referred to as elements in some cases) have recently been studied and developed intensively. Such an organic light-emitting element includes a pair of electrodes acting as an anode and a cathode, and at least one organic compound layers including a light-emitting layer between the pair of electrodes. Each organic compound layer has a thickness of several tens of nanometers. One of the pair of electrodes reflects light. The other electrode acts as a light extraction electrode. Light is partially reflected from the interface between the light extraction electrode and the external environment. Consequently, an optical interference effect occurs inside the organic light-emitting element to enhance the intensity of light having a specific wavelength.
Japanese Patent Laid-Open No. 2004-127795 discloses that optical interference can enhance the light extraction efficiency at a wavelength equal to the maximum peak wavelength of a spectrum desired to be extracted. FIG. 1 is a schematic sectional view of an organic light-emitting element disclosed in Japanese Patent Laid-Open No. 2004-127795. The organic light-emitting element shown in FIG. 1 includes a first electrode (anode) 11 doubling as a reflection layer, organic compound layers (including a hole injection layer 12, a hole transport layer 13, a light-emitting layer 14 and an electron transport layer 15) and a second electrode (anode) 16 made of a metal, in that order, on a substrate 10. Holes injected from the anode and electrons injected from the cathode are recombined in the light-emitting layer 14 by applying a current to the organic light-emitting element. The energy generated by the recombination excites the light-emitting material in the light-emitting layer, and when the light-emitting material returns to the ground state, the energy is emitted as light. Thus, the organic light-emitting element emits light.
This organic light-emitting element has a resonator structure on the substrate between the interface of the first electrode 11 and the hole injection layer 12 and the interface of the electron transport layer 15 and the second electrode 16. Let L be the optical length between the first electrode 11 and the second electrode 16, and let θ be the angle at which light from the element is viewed (when viewed at an angle perpendicular to the element, θ is 0°). Also, let φ (rad) be the sum of phase shifts of lights reflected from the interface between the first electrode 11 and the hole injection layer 12 and from the interface between the electron transport layer 15 and the second electrode 16, and let m be the order of optical interference. In this instance, the light having a wavelength λ (resonant wavelength) satisfying Formula 1 of the lights emitted from the light-emitting layer 14 can be intensified by the resonator structure.λ=2L·cos θ/(m−φ/2π) (m: natural number)  Formula 1
When emitted light is reflected from a reflection layer or an electrode in practice, however, the sum φ of the phase shifts is varied depending on the combination of the materials of the electrode and organic layer defining the reflection plane.
In Japanese Patent Laid-Open No. 2004-127795, the optical length of the resonator structure is set so that the resonator structure can intensify light at the maximum peak wavelength of a desired spectrum to be extracted from the element, in consideration of the position in the light-emitting layer 14 at which light can be most strongly emitted. By appropriately setting the maximum peak wavelength of a desired spectrum and the resonant wavelength, the intensity at the maximum peak wavelength of the desired spectrum can be increased.
It is preferable that the photoluminescence spectrum (PL spectrum) of the light-emitting material used in the above-described type of light-emitting element coincide with the spectrum of the desired color to be extracted from the element. This is because the resonant wavelength is set to a wavelength different from the maximum peak wavelength of the PL spectrum. If the PL spectrum is different from the spectrum of the desired color to be extracted, luminous efficiency may not be obtained sufficiently. However, light emitting materials are very rare which have PL spectra substantially the same as the spectrum of a desired color to be extracted from the element. Even such materials do not always satisfy properties other than emission color, such as lifetime and luminous efficiency.